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[From the Cambridge and Dublin Mathematical Journal, Vol. vi., p. 215
(November, 1851).]
i I
ON THE CONDUCTION OF HEAT IN CRYSTALS.
THE 21st, 22nd, and 23rd volumes of the Annales de Ghimie et de Physique contain three very interesting papers by M. de Senarmont, describing a series of experimental researches on the conduction of heat in crystals, as well as in bodies subject to mechanical pressure in one direction. The mode of experimental examination employed consisted in cutting a plate from the crystal to be examined, drilling a small hole through it near the middle, covering the faces with a thin coating of wax, and then heating the crystal by a wire or fine tube inserted into the hole. The heat caused the wax to melt in the neighbourhood of the hole, and thus a certain isothermal line was rendered visible to the eye, namely, the line corresponding to the temperature of melting wax. The variation of conductivity in different directions was indicated by the elliptical, or at least oval form of the line bounding the melted wax. This line remained sufficiently visible after the plate had cooled, and thus the eccentricity of the ellipse and the azimuth of its major axis could be examined at leisure. On allowing for errors of observation, it was found that, for a plate cut in a given direction from a given crystal, the axes of the ellipse had a determinate ratio, and the major axis a determinate azimuth. Universally it was found that the thermic corresponded with the crystallographic symmetry, so that for example in crystals belonging to the cubical system, the propagation of heat took place as it would have done in a homogeneous uncrystallized medium; in crystals belonging to the rhombohedral system the axis was a direction of thermic